1. Field of the invention
The present invention relates to a transimpedance amplifier circuit, and more particularly, to a transimpedance amplifier circuit with a broad frequency band, a high signal coupling efficiency, less foreign signals, and less background direct current (DC).
2. Description of the Prior Art
A conventional transimpedance amplifier circuit capable of isolating background DC is generally applicable for detecting a current signal. For example, a photosensor circuit which converting an optical signal into a current signal with a photodiode, and then converting the current signal to an output voltage signal by an amplifier is always employed as a receiving circuit in photo communication system. The main scheme thereof is shown in FIG. 1, wherein a coupling capacitor C is provided for an amplifier circuit 1 to prevent a DC current from flowing into the circuit 1. In this construction the coupling efficiency of a signal current Is is R1/ R1+Za, wherein Za is an equivalent impedance viewed from the signal terminal into the amplifier circuit 1. The output signal voltage will be Vs=Isxc3x97[ R1xc3x97R2/(R1+Za)], whereas output foreign signal voltage is Vn=Inxc3x97R2+Vn(1+R2/R1), wherein In is a foreign signal current small enough to be negligible. From the above derivation it is distinctly observable that a higher signal coupling efficiency and a lower foreign signal output voltage is obtainable from raising up the value of R1. However, if the background DC Ia is taken into account, wherein Iaxc3x97R1 less than Vcc, the amplifier circuit under consideration is still impossible to work under both too large Ra and Ia.
In order to eliminate the shortcomings inherent to the conventional circuit described above, the present inventor has delved into this matter with a long time efforts and came to realization of an improved scheme of a transimpedance amplifier circuit having the merits of a broad frequency band, a high signal coupling efficiency, less foreign signals, and less background DC.
It is an object of the present invention to provide a transimpedance amplifier circuit having the merits of a broad frequency band, a high signal coupling, efficiency, less foreign signals, and less back ground DC.
For achieving the above mentioned object, the transimpedance amplifier circuit according to the present invention consists of a buffer circuit, a simulation resistance circuit, and an amplifier circuit. The buffer circuit for inputting a signal current is constituted by two field effect transistors (FET) and a resistor, and has a high current input efficiency and function of widening circuit frequency band. The simulation resistance circuit is constituted by a resistor, two buffer units, a coupling capacitor, and a biasing resistor. When operating at a low frequency, the simulating resistance circuit permits a large amount of background DC to flow through; on the other hand, when operating at a high frequency, this circuit can improve the signal coupling efficiency and reduce foreign signal output voltage. On the whole, by means of the transimpedance amplifier circuit of the present invention, both the detecting sensitivity and the amplification factor of the signal current are greatly improved.